1. Field of the Invention
The present invention relates to a magnetic sensor for detecting a magnetic field by utilizing the tunneling magnetoresistive effect. More particularly, the present invention relates to a magnetic sensor, which is capable of providing a stable resistance change rate and can be formed with good reproducibility, as well as to a method for manufacturing the magnetic sensor.
2. Description of the Related Art
A GMR (giant magnetoresistive) sensor exhibiting the giant magnetoresistive effect is employed as a head mounted on, e.g., a hard disk drive and dedicated to reproduction, and it is known as having a high sensitivity.
Among various types of GMR sensors, there is a spin valve film having a relatively simple structure and having resistance capable of being changed by a weak external magnetic field. The spin valve film is of a four-layered structure in the simplest form.
FIG. 19 schematically partly shows a structure of a spin valve film. FIG. 19 is a sectional view looking from the side facing a recording medium.
In FIG. 19, reference numerals 1 and 3 each denote a ferromagnetic layer made of a NiFe alloy, for example. A nonmagnetic electrically conductive layer 2 made of Cu, for example, is interposed between the two ferromagnetic layers.
In the spin valve film of FIG. 19, the ferromagnetic layer 1 is the so-called free magnetic layer, and the ferromagnetic layer 3 is the so-called pinned magnetic layer. In this specification, the ferromagnetic layer 1 and the ferromagnetic layer 3 will be referred to as xe2x80x9cfree magnetic layerxe2x80x9d and xe2x80x9cpinned magnetic layerxe2x80x9d hereinafter, respectively.
Also, as shown in FIG. 19, an antiferromagnetic layer 4 made of a NiMn alloy, for example, is formed on the pinned magnetic layer 3 in contact with each other. With annealing a carried out under a magnetic field, an exchange anisotropic magnetic field is generated between the pinned magnetic layer 3 and the antiferromagnetic layer 4, whereby magnetization of the pinned magnetic layer 3 is pinned in the height direction (Y-direction as shown).
On the other hand, magnetization of the free magnetic layer 1 is affected by a bias layer (not shown), etc., and. aligned in the track-width direction (X-direction as shown). The magnetized directions of the pinned magnetic layer 3 and the free magnetic layer 1 are thus in a crossing relation.
A pair of electrode layers 5, 5 are provided, as shown in FIG. 19, on both sides of a multilayered film, including from the free magnetic layer 1 to the antiferromagnetic layer 4, as viewed in the track-width direction (X-direction). The electrode layers 5, 5 are formed of, e.g., Cu (copper), W (tungsten) or Cr (chromium).
In the spin valve film shown in FIG. 19, when the magnetized direction of the free magnetic layer 1 is varied depending on a leakage magnetic field from a recording medium such as a hard disk, electrical resistance is changed based on correlation to the magnetized direction of the pinned magnetic layer 3, whereby a voltage change is resulted depending on a change in value of the electrical resistance. In accordance with such a voltage change, the leakage magnetic field from the recording medium is detected. A resistance change rate (MR ratio) of the spin valve film is in the range of about several to ten-odd percentages.
Meanwhile, with the recent progress toward higher recording density, the plane recording density of a hard disk drive has been increased more and more. Such a trend raises a problem as to whether GMR sensors primarily used at present are adaptable for higher recording density expected in the future.
In that situation, attention has been focused on a tunneling magnetoresistive sensor as a reproduction head to be employed in place of GMR sensors. The tunneling magnetoresistive sensor has a resistance change rate (TMR ratio) as high as several tens of percent, and hence is able to produce a much greater reproduction output than that obtainable with the GMR sensors.
FIG. 20 schematically partly shows a structure of a conventional tunneling magnetoresistive sensor. FIG. 20 is a sectional view looking from the side facing a recording medium.
In FIG. 20, as with the spin valve film shown in FIG. 19, numerals 1 and 3 denote a free magnetic layer and a pinned magnetic layer, respectively. An antiferromagnetic layer 4 is formed on the pinned magnetic layer 3 in contact with each other.
The structure of the tunneling magnetoresistive sensor differs from that of the spin valve film primarily in the following points. An insulation barrier layer 6 made of Al2O3 (alumina), for example, is formed between the free magnetic layer 1 and the pinned magnetic layer 3. Also, a pair of electrode layers 5, 5 are provided on both sides of a multilayered film, including from the free magnetic layer 1 to the antiferromagnetic layer 4, as viewed in the vertical direction (Z-direction as shown) relative to surfaces of the multilayered film.
In the tunneling magnetoresistive sensor, when a voltage is applied to the two ferromagnetic layers (i.e., the free magnetic layer 1 and the pinned magnetic layer 3), an electric current (tunnel current) flows through the insulation barrier layer 6 based on the tunnel effect.
In the tunneling magnetoresistive sensor, as with the spin valve film, magnetization of the pinned magnetic layer 3 is pinned in the Y-direction as shown, and magnetization of the free magnetic layer 1 is aligned in the X-direction as shown. Then, the magnetized direction of the free magnetic layer 1 is varied under an influence of an external magnetic field.
When the magnetized directions of the pinned magnetic layer 3 and the free magnetic layer 1 are antiparallel to each other, the tunnel current is hardest to flow and the resistance value is maximized. When the magnetized directions of the pinned magnetic layer 3 and the free magnetic layer 1 are parallel to each other, the tunnel current is easiest to flow and the resistance value is minimized.
Upon the magnetized direction of the free magnetic layer 1 being varied under the influence of the external magnetic field, a resulting change in electrical resistance is taken out as a voltage change, whereby a leakage magnetic field from the recording medium is detected.
A resistance change rate (TMR ratio; xcex94RTMR) of the tunneling magnetoresistive sensor is expressed by 2PPPF/(1xe2x88x92PPPF). Herein, PP represents a spin polarization rate (i.e., difference in the number of electrons between upward spins and downward spins, which is normalized based on the total number of electrons; referred to simply as a xe2x80x9cpolarization ratexe2x80x9d hereinafter) of the pinned magnetic layer. PF represents a polarization rate of the free magnetic layer. As seen from that formula, the resistance change rate is determined depending on the polarization rate of the ferromagnetic layer. Theoretically, the resistance change rate is increased as the polarization rate increases.
In most of conventional tunneling magnetoresistive sensors, the insulation barrier layer 6 is formed of Al2O3 (alumina).
With the recent progress toward higher recording density, however, the following problems have occurred in the conventional tunneling magnetoresistive sensors using Al2O3 as a material of the insulation barrier layer 6.
(1) The first problem resides in dielectric strength. The insulation barrier layer 6 is formed in a very small thickness as thin as 1 to 2 nm. When the insulation barrier layer 6 is formed in such a very small thickness, the film made of alumina cannot provide a satisfactory level of dielectric withstand voltage. Hence, when a large electric current flows through the tunneling magnetoresistive sensor, the insulation barrier layer 6 is apt to cause breakdown.
In particular, the tunneling magnetoresistive sensor has a large sensor resistance, and when a voltage is applied to the two ferromagnetic layers (i.e., the free magnetic layer 1 and the pinned magnetic layer 3), the insulation barrier layer 6 is subjected to a fairly large voltage. Therefore, a probability of breakdown of the insulation barrier layer 6 is increased.
Also, the insulation barrier layer 6 must be made thinner in order to reduce the sensor resistance. With the dielectric withstand voltage lowered correspondingly, however, an electrical short-circuit is more likely to occur between the free magnetic layer 1 and the pinned magnetic layer 3. This leads to a difficulty in reducing the thickness of the insulation barrier layer 6.
(2) The second problem resides in developer resistance. In a process after forming the tunneling magnetoresistive sensor shown in FIG. 20, a main electrode layer and an inductive head are formed on the electrode layer 5. When forming the main electrode layer and the inductive head, the process includes a step of forming a resist pattern. However, alumina is apt to dissolve in a strong alkaline developer, for example, which is used in the step of forming the resist pattern. Also, the etching rate of alumina is very high when exposed to the developer.
Because the insulation barrier layer 6 is a very thin film as mentioned above, low developer resistance raises a difficulty in forming the insulation barrier layer 6 with certainty.
Further, a magnetic sensor is mounted on a trailing end surface of a slider made of, e.g., alumina titanium carbide (Alxe2x80x94Tixe2x80x94C). In a step of machining the slider after forming the magnetic sensor, the magnetic sensor is also exposed to liquids such as a cutting liquid and a lapping liquid. Because the insulation barrier layer 6 of the tunneling magnetoresistive sensor has a very small thickness, reliability of the product is greatly affected if, even though slightly, the insulation barrier layer 6 dissolves in those liquids or its properties change.
Conversely, to form the insulation barrier layer 6 having satisfactory levels of both dielectric withstand voltage and developer resistance, the film thickness of the insulation barrier layer 6 must be increased. This makes the tunnel current harder to flow.
(3) The third problem resides in heat radiation. A tunneling magnetoresistive sensor has high sensor resistance and generates a large amount of heat. Therefore, the insulation barrier layer 6 should have good heat radiation.
However, the insulation barrier layer 6 made of alumina cannot be said as having sufficient heat radiation. The sensor temperature is raised to such a level as adversely affecting characteristics.
Also, although alumina has relatively good smoothness, a material of the insulation barrier layer 6 is required to have better smoothness for forming the insulation barrier layer 6 as a very thin film.
Thus, the insulation barrier layer 6 made of alumina cannot satisfy required levels of all properties, i.e., dielectric strength, developer resistance, heat radiation, and smoothness.
With the view of overcoming the above-mentioned problems in the related art, it is an object of the present invention to provide a magnetic sensor and a method for manufacturing the magnetic sensor, in which an insulation barrier layer is formed of an Alxe2x80x94Sixe2x80x94O film or a Sixe2x80x94Oxe2x80x94N film for improvements in dielectric strength, developer resistance, heat radiation, and smoothness of the insulation barrier layer.
A magnetic sensor of the present invention comprises a multilayered film and electrode layers formed on upper and lower sides of the multilayered film, the multilayered film being made up of an antiferromagnetic layer, a pinned magnetic layer of which magnetized direction is pinned by an exchange coupling magnetic field developed between the pinned magnetic layer and the antiferromagnetic layer, and a free magnetic layer formed on the pinned magnetic layer with an insulation barrier layer interposed therebetween, wherein the insulation barrier layer is formed using an insulating material expressed by a composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N.
With the present invention, by using an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N to form the insulation barrier layer, dielectric strength, developer resistance, smoothness, and heat radiation of the insulation barrier layer can be improved.
Accordingly, in spite of the insulation barrier layer being formed as a very thin film in thickness of 0.3 to 2.0 nm, it is possible to obtain the insulation barrier layer that is stable and is less susceptible to damages by an electric current and chemicals.
Also, when the insulation barrier layer is formed using alumina as conventional, it has been in fact impossible to reduce the thickness of the insulation barrier layer down to a value smaller than 1 nm. In contrast, with the present invention, the insulation barrier layer can be formed in thickness smaller than about 0.5 nm. As a result, the DC resistance value of the magnetic sensor can also be reduced. More specifically, according to the present invention, the DC resistance value of the magnetic sensor can be increased to the order of several tens of ohms (xcexa9) which is comparable to those of conventional GMR sensors.
Hence, heat generation of the magnetic sensor can be suppressed and noise resistance of the magnetic sensor can be improved.
Addition of Si leads to another advantage that the insulation barrier layer, which is formed of an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O, has an improved dielectric withstand voltage in comparison with the insulation-barrier layer formed of alumina. As a result, a magnetic sensor can be provided in which breakdown is less likely to occur in the insulation barrier layer and a stable operation is ensured.
Further, the developer resistance of the insulation barrier layer is improved in comparison with the case of using alumina. While the etching rate of alumina is about 50 xc3x85/min, addition of Si reduces the etching rate of the Alxe2x80x94Sixe2x80x94O film from such a value. When the amount of Si added is about 9 at %, the etching rate is almost 0 xc3x85/min.
The magnetic sensor of the present invention can be used for forming a magnetic head of a hard disk drive. When forming a magnetic head of a hard disk drive, in a process after forming the magnetic sensor, a main electrode layer and an inductive head are formed on the electrode layer of the magnetic sensor. When forming the main electrode layer and the inductive head, the process includes a step of forming a resist pattern, and the magnetic sensor is exposed to a strong alkaline developer. With the present invention, however, since the insulation barrier layer is formed using an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N, the insulation barrier layer is hardly dissolved and the etching rate of the insulation barrier layer is small when exposed to the developer.
Further, the magnetic head is mounted on a trailing end surface of a slider made of, e.g., alumina titanium carbide (Alxe2x80x94Tixe2x80x94C). In a step of machining the slider after forming the magnetic head, the magnetic sensor is also exposed to liquids such as a cutting liquid and a lapping liquid. With the present invention, the insulation barrier layer is hard to dissolve in the liquids used for machining the slider, and the etching rate of the insulation barrier layer is small when exposed to those liquids.
Accordingly, in spite of the insulation barrier layer being formed as a very thin film, the insulation barrier layer can be reliably formed.
The reason why the dielectric strength and the developer resistance of an Alxe2x80x94Sixe2x80x94O film are improved as compared with those of an alumina film is presumably in that addition of Si in an insulating material made of Al and O improves both the dielectric strength and the developer resistance on the basis of coupling between Si and O.
It is also confirmed that the insulation barrier layer has smoothness substantially at the same level as that of an alumina film.
Additionally, the insulation barrier layer has better heat radiation than an alumina film and is able to suppress a rise of the sensor temperature satisfactorily even when the current density increases as a result of higher recording density expected in the future.
Hence, an increase in resistance value of the magnetic sensor can be suppressed.
The reason why the insulation barrier layer has better heat radiation than an alumina film is presumably in that the atomic array of an Alxe2x80x94Sixe2x80x94O film has regularity at a short range. An alumina film structure is in a completely amorphous state. On the other hand, in the Alxe2x80x94Sixe2x80x94O film, as the amount of Si added increases, regularity at a short range gradually occurs in the atomic array and crystallinity is improved.
Whether regularity at a short range occurs in the atomic array can be determined by observing a transmitted electron-beam diffraction image of a film.
From experimental results described later, it is found that the amount of Si added is preferably in a range of not less than 2 at % but not more than 9 at % with respect to a total.
Also, in the present invention, when Si in an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O is calculated in terms of SiO2 based on stoichiometry in relation to O, a calculated amount of SiO2 is preferably in a range of not less than 10 at % but not more than 38 at % in the insulating material.
Further, in the present invention, when Si in an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O is calculated in terms of SiO2 based on stoichiometry in relation to O, a calculated amount of SiO2 is preferably in a range of not less than 6.1 mass % but not more than 26.0 mass % in the insulating material.
The insulation material (barrier) layer has a thickness preferably in a range of not less than 0.3 nm but not more than 2.0 nm. More preferably, the insulation barrier layer has a thickness in a range of not less than 0.3 nm but not more than 1.0 nm.
Thus, according to the present invention, since the insulation barrier layer is formed of an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N, the insulation barrier layer is able to have improved dielectric strength, developer resistance, smoothness, and heat radiation. As a result, a magnetic sensor utilizing the tunneling magnetoresistive effect can be provided in which, in spite of the thickness of the insulation barrier layer being reduced, it is possible to obtain a stable resistance change rate and to form the insulation barrier layer with good reproducibility.
Further, the present invention provides a method for manufacturing a magnetic sensor comprising a multilayered film and electrode layers formed on upper and lower sides of the multilayered film, the multilayered film being made up of an antiferromagnetic layer, a pinned magnetic layer of which magnetized direction is pinned by an exchange coupling magnetic field developed between the pinned magnetic layer and the antiferromagnetic layer, and a free magnetic layer formed on the pinned magnetic layer with an insulation barrier layer interposed therebetween, wherein after forming an AlSi thin film or a SiN thin film, the AlSi thin film or the SiN thin film is oxidized naturally or oxidized in a radical oxygen gas, thereby forming the insulation barrier layer of an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N.
With the method of the present invention, since the AlSi thin film or the SiN thin film is oxidized naturally or oxidized in a radical oxygen gas, the AlSi thin film or the SiN thin film can be uniformly oxidized at an appropriate oxidizing rate.
The insulation barrier layer of an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N may be formed by using a target made of AlSi or SiN and depositing a film by sputtering while introducing an O2 gas to a sputtering apparatus.
With the above feature, since the target is made of AlSi or SiN, the Si amount can be set only based on a mixing ratio with respect to Al or N. Also, the Alxe2x80x94Sixe2x80x94O or Sixe2x80x94Oxe2x80x94N film having a predetermined composition can be easily formed by reactive sputtering while an O2 gas is introduced to a sputtering apparatus.
When forming the insulation barrier layer of an insulating material expressed by the composition formula of Alxe2x80x94Sixe2x80x94O, the Si content in the insulating material is preferably in a range of not less than 2 at % but not more than 9 at % with respect to a total. Further, the insulation barrier layer is preferably formed in thickness in a range of not less than 0.3 nm but not more than 2.0 nm. More preferably, the insulation barrier layer is formed in thickness in a range of not less than 0.3 nm but not more than 1.0 nm.